Answer
Choice B.
Work Step by Step
We’re told that the distance between particles is 10 times their size.
In Passage Problem BIO on page 1405, we are told that the radius of a Rydberg atom is approximately $1\;\mu m$, which means their size is $\approx 2\;\mu m$ and their separation is $\approx 20\;\mu m=20\times10^{-4}cm$.
Picture each atom being in a cubical box of volume $(20\times10^{-4}cm)^3$.
The particle density is the number of particles, divided by the volume they occupy.
$$\frac{1\;atom}{(20\times10^{-4}cm)^3}\approx\frac{10^8 atoms}{cm^3}$$
